sctp: remove unused union (sctp_cmsg_data_t) definition

This patch removes an unused union definition (sctp_cmsg_data_t)
from include/net/sctp/user.h.

Signed-off-by: Rami Rosen <rosenrami@gmail.com>
Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>